The interior trim appearance of an automobile has been a key area of marketing focus. Interiors are often given treatments which provide increases in perceived value as well as brand differentiation. Efforts to cut costs by removing even small interior trim details have met with significant adverse sales impact.
The current automotive interior styling trend is toward a softer, wrap-around styling vision which places special emphasis on the tactile feel of the interior, as well as the visual impression. The hard plastic substrates and surfaces of car interiors in past years are being replaced by the trend toward padded or cushioned surfaces, covered by either vinyl or textile materials. Accordingly, parts such as instrument panels, arm rests, center consoles, seating, head rests, door skins and body pillar trim have increasingly been provided with soft, padded interior trim for styling purposes, for noise abatement and for safety concerns.
The automotive interior surfaces just described typically comprise a structural substrate of aluminum, plastic or the like which is relatively rigid. A layer of padding and a trim panel may cover the interior surface of the substrate. The term "relatively rigid" as used herein refers to the fact that the substrate has greater stiffness than the trim panel covering it. Obviously some flexibility is found in components such as door panel substrates.
The surface of the trim panel which is exposed to the passenger is referred to as an A-side layer. Typical materials for the A-side layer are leather, vinyl or textile materials, including cloth or carpet. Vinyl materials often have a cotton knit backing and textiles usually have a latex backcoating applied by the fabric manufacturers to stabilize the cloth. While the materials used as A-side layers serve their cosmetic purposes well, none of them by themselves provide the soft, padded or cushioned effect desired, nor will they retain a shape. Accordingly, a layer of padding and structure must be provided between the substrate and the A-side layer.
Current interior trim suppliers are using a variety of methods to apply the A-side layer and padding to the substrates of various interior trim systems. The most common and least technologically advanced method is hand applying the coverings to the substrate. The layers are hand laid over a finished substrate and then pushed, pulled, tucked and pinched into the contour of the substrate. The layers are secured with adhesives and/or mechanical fasteners such as staples or the like. Edge finishing requires additional operations. Obviously hand applied trim panels have high labor costs. Accordingly much effort has been put into alternative methods of trim panel application such as vacuum forming and low pressure molding (LPM). In LPM the trim panel is attached concurrently with the molding of the rigid substrate. While vacuum forming has advantages over hand applying, it still has numerous drawbacks, including slow cycle times, tearing, stretching or thinning of the A-side layer and bridging of the A-side layer over grooves and the like. LPM avoids some of these problems but introduces new ones such as cost, additional trimming, part stability (warping and shrinkage), and uneven fill of the substrate (thin and thick spots). Like vacuum forming LPM suffers from slow cycle times.
What is needed is a trim panel that is padded and pre-formed to the contour of the substrate so it can be attached to the substrate by any method without the need for laborious hand contouring. None of the alternatives mentioned above addresses this need, in part because of the difficulties of forming any shape in A-side materials. The A-side materials on their own lack sufficient stiffness to retain any particular contour or shape, i.e., they will fold or collapse under their own weight.
A known technique for making formed parts is thermoforming. Thermoforming is defined as the forming of a thermoplastic sheet by heating it and then placing it into a mold to shape it. Cooling takes place in the mold so when the part comes out it will be selfsupporting, i.e., it will retain the shape or contour of the mold.
However, not all materials are thermoformable. A-side materials cannot be thermoformed effectively--again, they will not retain a shape or contour. Felt can be thermoformed, as can polypropylene. A-side materials have been combined with layers of felt and polypropylene to make thermoformed parts. But these parts do not have the padded or cushioned feel desired. Trim parts made of such materials would still require separately added layers of foam or other padding. Urethane and polyethylene foams by themselves are thermoformable and thermoformed parts made solely of these materials have been used in automotive applications. But these parts do not have the desired A-side surface treatment. So far as the inventors are aware, no one has ever made trim panels made of a thermoformable foam and an A-side material because there has been a perception that the technical difficulties of controlling the process were beyond reasonable. Specifically, the tendency of A-side materials to exhibit memory (retain their original flat shape and thereby resist molding) and to change their shape after thermoforming due to shrinkage makes them terrible candidates for thermoforming. Thus, A-side materials require a stout backing material to overcome these characteristics. While felt and polypropylene have sufficient rigidity when thermoformed to tame the ornery manners of thermoformed A-side materials, it was always assumed that foam materials would not have the required characteristics to combine with A-side materials. Plus, the danger of having a backing material that would stretch, tear or distort the A-side material counseled against the use of foam in conjunction with A-side materials.
Furthermore, lack of thermoforming process controls made the very notion of "precision" formed textiles and vinyls implausible in the automotive world. It was thought thermoformed textiles and vinyls could not be made in volume with precise, predictable and repeatable results. The common misconceptions among automotive textile people were twofold; first it was thought that the tough body cloths could not be molded as many of them are woven and have extremely well-developed wear characteristics, and secondly, the latex backcoating was so difficult to adhere to that the adhesives would release and the laminates would fail.
The present invention describes a material which overcomes these perceived drawbacks and provides a self-supporting, padded trim panel ready for attachment to a substrate in any desired manner. Process controls are provided which allow thermoforming these parts in quantity with precise, repeatable accuracy.